Rocket nozzle comprising pyrolytic graphite-silicon carbide microcomposite inserts

ABSTRACT

A rigid pyrolytic graphite microcomposite material comprising a matrix of pyrolytic graphite containing embedded therein codeposited crystalline silicon carbide comprising aciculae oriented approximately perpendicular to the a-b plane of the crystallite layers of the pyrolytic graphite. The SiC comprises at least about 5 volume percent of the microcomposite material, preferably at least about 10 volume percent. A rigid composite pyrolytic graphite article comprising a matrix of the above microcomposite material containing embedded therein at least one reinforcing refractory filament or strand layer. The refractory filament or strand layer comprises a plurality of unidirectional and substantially parallel, laterally spaced, individual, continuous refractory filaments or strands. The microcomposite matrix is nucleated from each of the individual refractory filaments or strands and interconnected to form a continuous matrix phase surrounding and interconnecting the individual filaments or strands comprising the embedded filament or strand layer.

mars-rubs SR United States Patent 1191 Olcott 1*Aug. 19, 1975 1 ROCKETNOZZLE COMPRISING PYROLYTIC GRAPHITE-SILICON CARBIDE MICROCOMPOSITEINSERTS Notice: The portion of the term of this patent subsequent toJune 12. 1990. has been disclaimed.

[22] Filed: Apr. 24, 1973 121] Appl No.: 354,048

Related US. Application Data [62] Division of Ser. No. 65.899. Aug. 21.1970. Pat. No.

[52] US. Cl. 428/367; 239/265.15; 239/591; 428/408". 428/373 [51] Int.Cl B32b 5/12 [58] Field of Search 161/168. 169. 206. 60; 117/D1G. 11,106 C. 169 A; 239/265.11. 265.15. 591

[561 References Cited UNITED STATES PATENTS 3.156.091 ll/l964 Kraus239/265.1l

3.317.356 5/1967 Clendinning 117/106 C 3.391.016 7/1968 McCrary etal.... 117/106 C 3.464.843 9/1969 Basche 1 17/106 C 3.629.049 12/1971Olcott 117/46 CG 3.653.851 4/1972 Gruber 117/106 C 3.676.293 7/1972Gruber 161/206 3.677.795 7/1972 Bokros et a1. 117/46 CG 3.685.059 8/1972Bokros et a1. 117/46 CG 3.738.906 6/1973 Olcott 161/57 Prinuu'yliraminerWilliam J. Van Balen Assistant E.\'uminerWilliam R. Dixon. Jr.Attorney. Agent. or FirmMartha L. Ross 5 7 ABSTRACT A rigid pyrolyticgraphite microcomposite material comprising a matrix of pyrolyticgraphite containing embedded therein codeposited crystalline siliconcarbide comprising aciculae oriented approximately perpendicular to thea-b plane of the crystalli'te layers of the pyrolytic graphite. The SiCcomprises at least about 5 volume percent of the microeompositematerial. preferably at least about 10 volume percent.

A rigid composite pyrolytic graphite article comprising a matrix of theabove microcomposite material containing embedded therein at least onereinforcing refractory filament or strand layer. The refractory filamentor strand layer comprises a plurality of unidirectional andsubstantially parallel. laterally spaced. individual. continuousrefractory filaments or strands. The microcomposite matrix is nucleatedfrom each of the individual refractory filaments or strands andinterconnected to form a continuous matrix phase surrounding andinterconnecting the individual filaments or strands comprising theembedded filament or strand layer.

17 Claims. 7 Drawing Figures ROCKET NOZZLE COMPRISING PYROLYTICGRAPHITE-SILICON CARBIDE MICROCOMPOSITE INSERTS CROSS REFERENCESTO'RELATED APPLICATIONS This application is a divisional applicationSer. No. 65.899 filed Aug. 21. 1970. now US. Pat. No. 3.738.906.

BACKGROUND OF THE INVENTION The superior high temperature and erosionresistant properties of rigid pyrolytic graphite materials are wellknown. These properties make the material particularly useful as linersfor chambers or vessels subject to such conditions, as rocket nozzleinserts. and the like.

Prolytic graphite. however. does have certain disadvantageous propertiesstemming from its particular crystallite structure and from its tendencyto oxidize. particularly at high temperatures in an odixizingatmosphere.

Pyrolytic graphite is normally produced by the pyrolysis of acarbonaceous gas. such as methane or propane. onto a heated substrate.Flat. hexagonal crystallitcs oriented parallel to the substrate surfaceare deposited in layers which build up into an essentially laminarstructure. The pyrolytic graphite crystal is considerably wider in itsflat or ab plane than along its thickness dimension or e-axis. As aresult. pyrolytic graphite is highly anisotropic in many of itsproperties. including strength. heat conductivity and thermal expansion.with attendant difficulties in practical use. As an example. thematerial has an exceedingly high coefficient of thermal expansion in thethickness or c-axis direction and a relatively low coefficient in the(I-l? direction. As a result. it is exceedingly difficult to match apyrolytic graphite liner or insert with a suitable backing materialwhich can avoid separation during thermal cycling. Because of itsweakness in the e-direction. due to its flat. plate-like and. thereby,laminar microstructure. pyrolytic graphite tends to delaminatc underhigh stresses.

The embedding within the laminar pyrolytic graphite crystallitestructure of aciculae of crystalline SiC which are oriented in theedireetion. as compared to the planar orientation of the layers of thepyrolytic graphite in the a-h direction. advantageously reduces in themicrocomposite the anisotropic effect of the matrix and reduces thetendency of the graphite to delaminate. Additionally. it substantiallyimproves oxidation resistance since. unlike carbon which oxidizes to agas. silicon oxidizes to SIOg which fuses to form a protective coating.Improved oxidation-resistance is particularly important if the pyrolyticgraphite is exposed to high temperature oxidative atmospheres.

The production of SiC films and coatings. for exam ple. on flexiblemetal filaments such as tungsten, by vapor phase pyrolysis of a silane.such as SiH SiCl SiHCl (CH Si or CH;,SiCl with or without addedhydrocarbon gas. is well known. the objective generally being theproduction of pure SiC. The pyrolysis temperatures employed aregenerally below the optimum temperatures for producing pyrolyticgraphite.

Seishi Yajima et al.. Journal of Materials Science 4 (1969) pp. 416-423and 424-431. and Chemical Ab Sll'tICIS. 1970. 7. p. 69. disclose astructure comprising flake-like single crystals of SiC dispersed in amatrix of pyrolytic graphite and oriented parallel to the planes of thegraphite. The crystallite size of the SiC was about 200 A thick(c-direction) and about 2000 A in diameter (ll-l7 direction). Since thesingle SiC crystals of the Yajimaiet al. structures are essentially flatand oriented in the same planar direction as the pyrolytic graphite.they cannot have any substantial, effect on the anisot ropy ordelamination characteristics of thelatter.

Yajima et al. pyrolyzed a mixturcof SiCh and pro- 7 pane under vacuum.Maximum SiC production of up to 4 weight percent was obtained attemperatures of about 1400 to 1500c and dropped to as little as 0.02 to0.03 weight percent at temperatures of about 2000C. Since SiC isconsiderably denser than pyrolytic graphite. the volume percent of SiCwas substantially smaller.

None of the referenced art discloses the pyrolytic graphite-SiCmicrocomposite of this invention or the process for making it.

Copending applications Ser. No. 592.846 and 870.948. now US. Pat. Nos.3.629.049 and 3.715.253 respectively. disclose rigid pyrolytic graphitearticles comprising a matrix ofpyrolytic graphite containing embeddedtherein at least one reinforcing layer consisting ofa plurality ofunidirectional and substantially parallel. laterally spaced. individual.continuous carbon strands. The matrix comprises crystallite layers ofpyrolytic graphite nucleated from each of the individual carbon strandsand interconnected to form a continuous phase surrounding andinterconnecting the individual strands comprising the embedded strandlayers. By conforming the crystallite pyrolytic graphite layers toembedded strand surfaces instead of to the surface of a conventionalbase substrate. anisotropy of the pyrolytic graphite and its attendantdisadvantages are substantially reduced.

Utilization of the codeposited pyrolytic graphite-SiC microcomposite ofthe present invention in place of the pyrolytic graphite matrixdisclosed in said copcnding applications provides further improvement inisotropy and improves oxidation resistance.

The object of the invention is to provide a rig id pyro lyticgraphite-SiC microeomposite having substantially lower anisotropy thanpyrolytic graphite and improved oxidation resistance.

Still another object is to provide a process for making said rigidpyrolytie graphiteSiC microcomposite.

Another object is to provide rigid reinforced composite pyrolyticgraphite-SiC articles having additionally decreased anisotropy.

Still another object is to provide a process for making said rigidreinforced composite pyrolytic graphite-SiC articles.

Other objects and advantages will become apparent from the followingdescription and drawings.

SUMMARY OF THE INVENTION Broadly. the invention comprises rigidmicrocomposite pyrolytic graphite materials containing codeposited andembedded therein crystalline SiC comprising aciculae. the longitudinalaxes of which are oriented approximately perpendicular to the 11-17 orflat plane of the pyrolytic graphite crystallite layers. Themicrocomposite is a two-phase system since the pyrolytic graphite andSiC are mutually insoluble.

The codeposition of aciculae of SiC within a matrix of pyrolyticgraphite in such manner that the longitudinal axes of the aciculae areoriented approximately in the e-direction relative to the ub plane ofthe pyrolytic graphite provides a substantial dimension in the thicknessor c-dircction which considerably reduces the anisotropy normallycharacteristic of pyrolytic graphite alone. This results insubstantially increased strength in the thickness dimension andimprovement in other propertiesfsuch as thermal expansion. Additionally.the perpendicularly embedded SiC aciculae interrupt the laminar patternof the pyrolytic graphite and thus reduce its tendencyto delaminate.Since SiC is considerably harder than pyrolytic graphite. the presenceof the former in the microcomposite also improves erosion-resistance. aswell as the oxidation resistance of the graphite.

Theh composite pyrolytic graphite-SiC material can be prepared bypyrolyzing a mixture of methyl trichlorosilane and a hydrocarbon gasonto a heated substrate at temperatures of about 2800 to 4()()OF.preferably about 3200 to 38()()F. in a suitable furnace in accordancewith procedures otherwise well known in the production of pyrolyticgraphite.

The invention additionally comprises rigid composite articles comprisingthe aforedescribed pyrolytic graphite-SiC microcomposite containingembedded therein at least one reinforcing layer of a plurality ofunidirectional and substantially parallel. laterally spaced. individual;continuous refractory filaments or strands. The pyrolytic graphite-SiCis nucleated from each of the individual refractory filaments or strandsand is interconnected to form a continuous matrix phase surrounding andinterconnecting the individual filaments or strands comprising theembedded filament or strand layer.

Nucleation and growth of the pyrolytic graphite-SiC microcomposite fromthe embedded plurality of refractory filaments or strands furtherreduces and interrupts the laminar character of the pyrolytic graphiteportion of the composite material and thereby further reduces anisotropyand delamination tendency. Additionally, the reinforcing refractoryfilaments or strands increase the strength of the composite article inthe direction of filamenfor strand orientation The rigid reinforcedcomposite pyrolytic graphite- SiC'articl'es can be made by progressivelypositioning a continuous, individual refractory filament or strand ontoa shaped form and simultaneously pyrolyzing a mixture of methyltrichlorosilane and a hydrocarbon gas onto the filament or strand atabout the point of positioning contact to nucleate pyrolytic graphiteand silicon carbide from the filament or strand. progressivelypositioning additional filament or strand laterally spaced frompreviously positioned filament or strand and. as the additional filamentor strand is positioned, simultaneously pyrolyzing the mixture of methyltrichlorosilanc and hydrocarbon gas thereon at about the point ofpositioning contact and on the codeposited pyrolytic graphite and SiCnucleated from previously positioned filament or strand. The pyrolysistemperature should be about 28()() to 40()()F. preferably about 3200 to380()F.

DRAWINGS FIG. 1 is a photomicrograph at a magnification of 150 of across-section of a sample of the pyrolytic graphite-SiC microcompositeof the invention.

FIG. 2 is a photomicrograph of the same section at a magnification of600.

FIG. 3 is a schematic illustration of apparatus for practicing thisinvention.

FIG. 4 is a schematic illustration of a rigid filamentorstrand'reinforced pyrolytic graphite-SiC composite according to thisinvention.

FIGS. 5 and 6 are schematic representations of modi fied apparatussuitable for use in preparing the filamentor strand-reinforcedcomposites.

FIG. 7 schematically illustrates an alternative arrangement ofreinforcing strands.

DETAILED DESCRIPTION The amount of SiC should be at least about 5 percent. preferably at least about 10 percent. by volume of themicrocomposite. Depending upon the desired properties for a particularapplication. the precent of SiC can be as high as 90 or even 95. Ingeneral. the preferred range is about 10 to 50 volume percent, with thepyrolytic graphite making up the remainder.

In some applications. it may be desirable to use a microcomposite ofgraded relative pyrolytic graphite and SiC composition. For example. theoutermost portion of the microcomposite can have a higher SiC content tominimize oxidative surface erosion. Such graded variations in therelative amounts of the codeposited pyrolytic graphite and SiC canreadily be achieved by varying respective flow rates of the methyltrichlorosilane and hydrocarbon gas and/or other processing variables inthe codeposition process.

The photomicrographs of FIGS. 1 and 2 at 150x and 600x magnificationrespectively. clearly show the SiC. a large proportion of which is inthe form of needle-like aciculae of SiC oriented substantiallyperpendicularly to the codeposited laminar layers of pyrolytic graphite.which forms an embedding matrix. The volume percent in the photographedsample is about percent.

The microcomposite can be made by vapor phase pyrolysis of a mixture ofmethyl trichlorosilanc and a hydrocarbon gas onto a heated substrate ata temperature of about 2800-4000F. preferably about 32003800F. An inertdiluent gas. such as argon. nitrogen. helium. hydrogen. and mixturesthereof is generally desirable. with some or all of the gas used toaspirate the liquid methyl trichlorosilane. Mixtures of hydrogen withargon. helium or nitrogen has been found 7 particularly effective inobtaining good aciculae crystalline SiC formation. The process can becarried out in a conventional furnace and related equipment at reducedor atmospheric pressures. Atmospheric pressure is generally preferredbecause of the excellent results obtained and the convenience.

The relative flow rates of the methyl trichlorosilane and hydrocarbongas vary generally with the desired microcomposite composition. Ingeneral. the silane may be introduced at a weight percent flow rate ofabout 5 to 75 percent. preferably about 15 to 50 percent and thehydrocarbon gas at a weight percent flow rate of about to 95 percent.preferably about 15 to percent.

The hydrocarbon gas can be any of those generally employed in producingpyrolytic graphite by vapor phase deposition. such as the lower alkanes.e.g. methane. ethane. and propane; ethylene; acetylene; and

mixtures thereof. Methane is preferred.

EXAMPLE I 5 hydrogen were injected into one end of the graphitecylinder. The methyl trichlorosilane was entrained for injection bybubbling argon through a container of the liquid methyl trichlorosilane.Flow rates were: argon l3 std. cu. ft/hr; hydrogen l(l std. cu. ft/hr;methane 2.0 std. cu. ft/hr. i

Total methyl trichlorosilane consumed was 85 gm.

Pyrolytic deposition was continued for 1 hour.

The thickness of the formed microcomposite and the relative amounts ofthe codeposited pyrolytic graphite and silicon carbide varied withdistance from the injection nozzle. The thickest portion of themicrocomposite formed was 26 mils and contained about 25 volume percentof needle-like crystalline aciculae of silicon carbide embedded inlaminar layers of pyrolytic graph ite. The volume percent of siliconcarbide decreased with increasing distance from the injector. Thephotomicrographs of FIGS. 1 and 2 were made with a sample taken from adownstream portion having a silicon carbide volume percent of about 20.

The rigid microcomposite cylinder formed by the above procedure wassound and showed no signs of delamination after cooling.

EXAMPLE II A run was made under conditions substantially the same as inExample I except that the pyrolysis temperature was maintained at36()()F.

Results were substantially similar except that at the point of maximumdeposition, the relative volumes of the SiC aciculae and the pyrolyticgraphite were 15 to 85 percent and then decreased with increasingdistance from the injector.

The rigid microcomposite cylinder was sound and sho\ 'cd no signs ofdelamination after cooling.

EXAMPLE III A pyrolytic graphite-SiC microcomposite was deposited on a1-inch diameter disc in a manner similar to the procedure used in thepreceding examples except that no hydrogen was used and a l-inch discsubstrate was centered at right angles to the injector so that asubstantially uniform microcomposite was formed over the face of thedisc.

To determine oxidation resistance. the resulting pyrolytic graphitc-SiCmicrocomposite disc and a disc of the same size and substrate coatedwith an equal thickness of pyrolytic graphite were heated to about 3000Fin a highly oxidizing oxyacetylene flame for 3 minutes. The pyrolyticgraphite coating was fully penetrated and almost completely burned awaywhereas the pyrolytic graphite-Sic coating eroded only on the surfacewith almost half of the thickness remaining intact.

EXAMPLE lV Several pyrolytic graphite and pyrolytic graphitc-SiCmicrocomposite deposition runs were made on ATJ graphite discs whichhave a higher coefficient of thermal expansion than pyrolytic graphitein its u-l) plane. By cross-sectioning of the deposits. it wasdetermined that all of the microcomposites were free from dclamination.whereas the pyrolytic graphite deposits showed major dclaminationsbetween the deposit and the substrate.

The pyrolytic graphite-SiC microcomposites can be reinforced to increasestrength and further reduce anisotropy of the pyrolytic graphitecomponent by embedding at least one layer of a plurality ofunidirectional and substantially parallel. laterally spaced. individualcontinuous. refractory filaments or strands in the microcomposite bynucleating the codeposited pyrolytic graphite and SiC from each of thefilaments or strands to form a continuous interconnecting matrixsurrounding and interconnecting the individual filaments or strands.

The strands or filaments can comprise any suitable refractory materialsuch as carbon in any suitable form including. for example. pyrolyzedrayon and pyrolytic graphite; SiC-coated metal filaments. such astungsten; carbon alloyed with a metal. such as Th. W. Ta. Mb. or Zr. inamounts. for example. up to about 20 percent by weight; boron filaments.and the like.

The method can be practiced with apparatus such as that schematicallyillustrated in H0. 3. As shown therein. a continuous. individualrefractory filament or strand. as for example carbon strand. 1. is fedthrough a guide tube 2. and connected to a mandrel 3. disposed inchamber 4. To prevent oxidation of the carbonaceous gas. atmosphericoxygen is removed and continuously excluded from the chamber byevacuation and/or purging with inert gases such as helium or nitrogen.The strand is heated to and maintained at a temperature sufficient topyrolyze the methyl trichlorosilane and hydrocarbon gases by induction.radiant. or resistance heating means. not shown. The mandrel is rotatedand moved longitudinally relative to the strand guide tube 2. by meansnot shown. In this manner. spaced turns of strand are progressivelypositioned on the mandrel. As the strand is wound. the methyltrichlorosilane. hydrocarbon and carrier gas mixture are fed throughtube 5. to impinge upon the strand at about the point of windingcontact. Pyrolysis of the methyl trichlorosilane and hydrocarbon gasoccurs and a pyrolytic graphite-Sic microcomposite matrix is nucleatedfrom the heated strand substrate. As winding continues. themicrocomposite is simultaneously deposited on the strand being wound andon the matrix deposited on previously wound strands. Thus. the strandsare not only individually enveloped in a microcomposite matrix but areinterconnected and bonded to each other by the matrix. The winding iscontinued to produce a composite article such as schematicallyillustrated in FIG. 4. As shown. the article comprises one or morespaced. reinforcing strand layers 6. each of which comprises a pluralityof spaced strands l. disposed in and interconnected by a pyrolyticgraphiteSiC microcomposite matrix 7. composed of graphite crystallitelayers 8 containing embedded. perpendicularly oriented. codepositedaciculae of SiC.

As shown. the crystalline layers of the pyrolytic graphite in themicrocomposite matrix are oriented in conformity to surfaces of thestrands and are. therefore. aligned around the strands and in thedirection of strand orientation. thereby maximizing strength of thepyrolytic graphite component in that direction. Furthermore. theembedded strands significantly reinforce the microcompositcstrandcomposite in the direction of strand orientation.

Since the orientation of the pyrolytic graphite crystallite layersconforms to the strand surfaces rather than the base or mandrelsubstrate surface of the composite. the pyrolytic graphite component ofthe microcomposite does not have the continuous laminar structurecharacteristic of conventional pyrolytic graphite. This, together withthe embedded codeposited SiC aciculae, further tends to preventpropagation of cracks and delaminations. Composite strength in thethickness direction is also further significantly improved by theincreased degree of crystallite layer alignment in that direction. Inaddition, the marked disparity in thermal expansion in the ab and cdirections characteristic of conventional pyrolytic graphite is furtherreduced.

The strands also prevent delamination failures by restricting thethickness of laminar pyrolytic graphite component growth units nucleatedfrom these strands. It is known that growth units less than 0.05 inchesthick are less subject to delamination. Since, in the composi tion ofthis invention, the thickness of laminar pyrolytic graphite componentunits is generally about one-half the distance between the strands;preferred unit size is obtained by spacing the strands less than 0.1inch of each other.

The process for composite fabrication can be practiced with individualstrands, as in the embodiment described, or with multi-strandstructures, such as a plurality of laterally spaced, unidirectionallyoriented individual strands, or with woven cloths or tapes comprisingstrands oriented in both warp and woof directions. When usingmulti-strand structures to prepare a composite, it is preferredsimultaneously to impinge the reactive gas mixture on both sides of thestrand structure as it is progressively laid down to ensure that the gaspenetrates between the strands to effect the highest degree of lateralbonding. This can be accomplished by apparatus such as schematicallyillustrated in FIG. 5, wherein gas injector channels 9, feed gas intocontact with spaced strands l, or by apparatus as shown in FIG. 6,wherein woven refractory cloth 11 and gas are both fed through guidechannel 10.

When the method is practiced with woven fabrics, little matrix bond isobtained between strands where warp and woof intercross since it isdifficult for the reaction gas mixture to penetrate between the touchingstrands. It is, therefore, preferred that all strands in eachreinforcing strand layer in the composite be substantiallyunidirectionally oriented. Such orientation eliminates weaknesses whichresult from the absence of a matrix bond at points of strand to strandcontact. In composites having multiple reinforcing strand layers, thedirection of strand orientation can be varied in different reinforcinglayers as shown, for example, in FIG. 7. Thus composites having desireddirectional strength characteristics can readily be prepared.

This invention can, of course, be practiced by positioning strand on avariety of shaped forms to produce articles having the desiredconfiguration. The strand can be progressively positioned on the shapedform by any desired technique. However, winding is preferred for reasonsof simplicity. It will be understood from the foregoing discussion thatthe term progressively positioning connotes a gradual laying down ofstrand to continuously and progressively increase the area of strandcontact with the shaped form rather than effecting overall lateralstrand contact as by stacking." This permits matrix formation betweenstrands as they are positioned and eliminates the necessity of forcingthe feed gas mixture between prepositioned strands.

When the invention is practiced with strands, such as carbon yarns,which comprise a multiplicity of fibers which have been spun orotherwise incorporated to form the continuous strand, the pyrolyticgraphite-SiC microcomposite may, in some instances be deposited onfibers or fuzzprotruding from the strand rather than directly on thebase strand. Therefore, in order to obtain optimum lateral bonding ofstrands by the matrix, it may be desirable to minimize such protrusionsas, for example, by mechanically removing them with a scraper blade asthe matrix is built up or by utilizing strands precoated with pyrolyticgraphite to provide a smooth surface.

Although this invention has been described with reference toillustrative embodiments thereof, it will be apparent to those skilledin the art that the principles of this invention can be embodied inother forms but within the scope of the claims.

I claim:

1. Rocket nozzle insert of pyrolytic graphite microcomposite comprisingpyrolytic graphite containing embedded therein codeposited siliconcarbide compris ing aciculae of silicon carbide, the longitudinal axesof said aciculae being aligned in the c-direction relative to the 11-12plane of the associated pyrolytic graphite crystallite at the point ofembedment, said silicon carbide comprising about 5 to percent by volumeof said microcomposite, said insert being substantially cylindrical.

2. The rocket nozzle insert of claim 1 in which the silicon carbide insaid microcomposite comprises at least 10 percent by volume of saidmicrocomposite.

3. The rocket nozzle insert of claim 2 in which the silicon carbidecomprises about ID to 50 percent by volume of said microcomposite.

4. The rocket nozzle insert of claim 1 in which the microcompositecomprises a matrix containing embedded therein at least one reinforcingrefractory filament or strand layer, said filament or strand layercomprising a plurality of unidirectional and substantially parallel,laterally spaced, individual, continuous refractory filaments orstrands, said matrix comprising crystallite layers of said pyrolyticgraphite-silicon carbide microcomposite nucleated from each of saidindividual filaments or strands and interconnected to form a continuousmatrix phase surrounding and interconnecting each of said individualfilaments or strands comprising said embedded at least one filament orstrand layer.

5. The rocket nozzle insert of claim 4 wherein said at least onereinforcing layer comprises a plurality of layers.

6. The rocket nozzle insert of claim 5 wherein the unidirectional,substantially parallel filaments or strands comprising at least onefilament or strand layer are oriented in a direction different from theunidirectional, substantially parallel filaments or strands comprisingat least one other filament or strand layer.

7. The rocket nozzle insert of claim 4 wherein the refractory strand iscarbon.

8. The rocket nozzle insert of claim 5 wherein the refractory strand iscarbon.

9. The rocket nozzle insert of claim 1 which includes a graphitesubstrate onto which said microcomposite is deposited.

10. The rocket nozzle insert of claim 3 which includes a graphitesubstrate onto which said microcomposite is deposited.

11. The rocket nozzle insert of claim 5 wherein the silicon carbide insaid microcomposite matrix comprises at least about l percent by volumeof said matrix.

12. The rocket nozzle insert of claim 11 wherein the silicon carbide insaid microcomposite matrix comprises about to 50 percent by volume ofsaid matrix.

13. The rocket nozzle insert of claim 6 wherein the silicon carbide insaid microcomposite matrix comprises at least about 10 percent by volumeof said matrix.

14. The rocket nozzle insert of claim 13 wherein the refractory strandis carbon.

1. ROCKET NOZZLE INSERT OF PYROLYTIC GRAPHITE MICROCOMPOSITE COMPRISINGPYROLYTIC GRAPHITE CONTAINING EMBEDDED THEREIN CODEPOSITED SILICONCARBIDE COMPRISING ACICULAE OF SILICON CARBIDE, THE LONGITUDINAL AXES OFSAID ACICULAE BEING ALIGNED IN THE C-DIRECTION RELATIVE TO THE A-B PLANEOF THE ASSOCIATED PYROLYTIC GRAPHITE CRYSTALLITE AT THE POINT OFEMBEDMENT, SAID SILICON CARBIDE COMPRISING ABOUT 5 TO 95 PERCENT BYVOLUME OF SAID MICROCOMPOSITE, SAID INSERT BEING SUBSTANTIALLYCYLINDRICAL.
 2. The rocket nozzle insert of claim 1 in which the siliconcarbide in said microcomposite comprises at least 10 percent by volumeof said microcomposite.
 3. The rocket nozzle insert of claim 2 in whichthe silicon carbide comprises about 10 to 50 percent by volume of saidmicrocomposite.
 4. THE ROCKET NOZZLE INSERT OF CLAIM 1 IN WHICH THEMICROCOMPOSITE COMPRISES A MATRIX CONTAINING EMBEDDED THEREIN AT LEASTONE REINFORCING REFRACTORY FILAMENT OR STRAND LAYER, SAID FILAMENT ORSTRAND LAYER COMPRISING A PLURALITY OF UNIDIRECTIONAL AND SUBSTANTIALLYPARALLEL, LATERALLY SPACED, INDIVIDUAL, CONTINOUS REFRACTORY FILAMENTSOR STRANDS, SAID MATRIX COMPRISING CRYSTALLITE LAYERS OF SAID PYROLYTICGRAPHITESILICON CARBIDE MICROCOMPOSITE NUCLEATED FROM EACH OF SAIDINDIVIDUAL FILAMENTS OR STRANDS AND INTERCONNECTED TO FORM A CONTINUOUSMATRIX PHASR SURROUNDING AND INTERCONNECTING EACH OF SAID INDIVIDUALFILAMENTS OR STRANDS COMPRISING SAID EMBEDDED AT LEAST ONE FILAMENT ORSTRAND LAYER.
 5. The rocket nozzle insert of claim 4 wherein said atleast one reinforcing layer comprises a plurality of layers.
 6. Therocket nozzle insert of claim 5 wherein the unidirectional,substantially parallel filaments or strands comprising at least onefilament or strand layer are oriented in a direction different from theunidirectional, substantially parallel filaments or strands comprisingat least one other filament or strand layer.
 7. The rocket nozzle insertof claim 4 wherein the refractory strand is carbon.
 8. The rocket nozzleinsert of claim 5 wherein the refractory strand is carbon.
 9. The rocketnozzle insert of claim 1 which includes a graphite substrate onto whichsaid microcomposite is deposited.
 10. The rocket nozzle insert of claim3 which includes a graphite substrate onto which said microcomposite isdeposited.
 11. The rocket nozzle insert of claim 5 wherein the siliconcarbide in said microcomposite matrix comprises at least about 10percent by volume of said matrix.
 12. The rocket nozzle insert of claim11 wherein the silicon carbide in said microcomposite matrix comprisesabout 10 to 50 percent by volume of said matrix.
 13. The rocket nozzleinsert of claim 6 wherein the silicon carbide in said microcompositematrix comprises at least about 10 percent by volume of said matrix. 14.The rocket nozzle insert of claim 13 wherein the silicon carbide in saidmicrocomposite matrix comprises about 10 to 50 percent by volume of saidmatrix.
 15. The rocket nozzle insert of claim 6 wherein the refractorystrand is carbon.
 16. The rocket nozzle insert of claim 12 wherein therefractory strand is carbon.
 17. The rocket nozzle insert of claim 14wherein the refractory strand is carbon.